This invention relates to drying particles derived from wood such as sawdust, wafers, flakes, paper sludge, or strands and more particularly to a method and apparatus of drying wood strands in which the amount of volatile organic compounds (VOC""s) produced during the drying process is reduced to an amount significantly below EPA maximum allowable requirements. The invention described herein achieves the desired reduction in VOC""s using significantly less energy than is possible with existing technology.
The wood particles referred to above commonly vary in size from a 0.005xe2x80x3 diameter dust particle to a wood strand or flake generally 0.030-0.050 inches thick, approximately 1 inch wide and varying in length up to 15xe2x80x3 long. The longer strands are typically used in making particle board, oriented strand board or similar wood construction products. In the case of sludge, the particles are more globular in shape with varying cross-sections and dimensions. In all instances, the materials cannot be used to make the different wood products because of their moisture content. The moisture content of sludge particles is typically 65% or greater (on a wet basis). Particles derived from wood pieces typically have moisture contents of 40-50% on a wet basis. Because they cannot be used when their moisture content is so high, they are subjected to a drying process by which the moisture content is reduced to approximately 2%.
Various wood drying processes are known in the art. A traditional process involves placing the wood strands in a multi-pass dryer. In this process, fuel is burned in a combustion chamber to produce a hot gas which is circulated through the dryer so to heat the wood strands to a temperature sufficiently high to drive out the moisture in the strands. Alternatively, the hot gases are used to heat oil, which is pumped to heat exchangers where air is heated. The hot air is then used for drying the wood, the net result in the dryer being the same.
An additional process consists of loading strands onto a conveyor belt or the like and moving them through the dryer at a speed slow enough to insure that the strands are heated to a desired temperature. The dryer can be a multi-pass type dryer in which instance the conveyor either follows a serpentine path through the dryer so the wood strands are exposed to heat for a long period of time, or the strands are off-loaded from one conveyor, after exiting the dryer, and then onto another conveyor which is routed back through the dryer. It will be understood that in a multi-pass drying system, the conveyor may have three or four segments extending through the dryer, or the strands may be moved to three or four separate conveyors as part of the drying process.
An alternate drying process is shown, for example, in U.S. Pat. No. 5,588,222. Here, there is a first drying stage involving one dryer, and second drying stage involving two or more separate dryers. Each dryer has its own separate fuel combustion chamber with which it is coupled. All of the wood strands are fed through the first drying stage and heated by a hot gas produced by the first combustion chamber. At the end of this first stage, the wood strands and hot gas are separated from each other. A portion of the wood strands are then routed to the dryers comprising the second drying stage. A portion of the hot gas is now recirculated to the first combustion chamber, while the remainder of the gas is routed to the burners and combustion chambers for the other dryers. There the hot gas is combined with other heated gases produced by the respective combustion chambers for the second stage dryers and blown over the strands being moved through the respective second stage dryers. At the end of the second stage dryers, the wood strands are fed off to their next process stage, and the hot gases are vented.
Among the several objects of the present invention may be noted the provision of apparatus for drying wood particles or strands having a high moisture content. The apparatus comprises a two-stage drying system specifically designed to significantly reduce VOC""s produced while reducing the moisture content of wood particles or strands processed through the system. The apparatus requires significantly overall less energy (dehydration and VOC abatements) than current technology. The first stage of the system employs a rotary dryer into which wood pieces are introduced with hot gases produced by a burner and combustion chamber circulating through the rotary dryer as the wood passes through it. After removal from the rotary dryer, the wood pieces and gases are separated with the wood pieces being directed to the inputs of one or more second stage dryers as determined by the mass flow requirements for the system. The gases and VOC""s are recycled back to the combustion chamber and re-burned. This re-burning, which occurs at a high temperature breaks down the VOC""s into constituent gases which are readily combusted. This reduces the amount of VOC""s generated during the drying process to a level which meets or is less than the EPA maximum allowed. Hot gases for further drying of the wood pieces in the second stage dryer(s) are supplied form the same combustion chamber which supplies the hot gases to the first stage dryer so only one combustion chamber is required to provide all of the heating requirements of the system. At completion of the second stage of drying, the wood pieces leave the dryer with most of their moisture removed so the wood strands are now available for use in other processes.